I'm sure theres an easy way to do this, but the most 'interfacy' way to do so would be to use a trigger zone at the top.
Anyway, here goes (UNTESTED METHOD);
1) Place the anti gravity platform in the map. Change the script to lift1.fpi. Set its name to 'Platform'.
2) At the exit to the platform (where the player gets off) place a Trigger Zone. Set the script of the Trigger Zone to plrinzoneactivateused.fpi.
3) Set the Trigger Zone's If Used field to 'Platform'
Alternatively there is scripting, so if that doesn't work I'll try and make one for you
Keep thinkin' outside the box,
JRH